A new approach to chemical synthesis of polynucleotides on solid supports is under investigation. The approach involves the synthesis of fully protected oligonucleotide blocks using the barium salts of protected deoxyribonucleoside-3' p-chlorophenyl phosphates as the key monomer intermediates. The blocks are then added sequentially to the growing polynucleotide strand attached to a glass support. This methodology is being used to prepare relatively large quantities of specific defined sequence polynucleotides for studies of secondary structures that may be formed in single-stranded DNA.